[sonic-xcvrd] add new daemon sonic-xcvrd to platform monitor (sonic-n…

…et#17)

* [sonic-xcvrd] add new daemon sonic-xcvrd to fetch Transceiver informations to DB
* This new daemon will periodcally read DOM information from eeprom and post to DB
* This new daemon will listen to the SFP plug in/out event and update the DB accordingly
* fix typo
* revise according to comments
* add tx_power support
* remove unnecessary try/except
* add error handle for get wrong status from common API
* fix review comments
* rename the event, move event and timer inside the class
* code readability fix